Large amounts of low rank coals, i.e. sub-bituminous coal, lignite and peat, from the Western United States are burned for electricity generation. Inorganic material contained in this coal produces particulate matter, such as fly ash, during its combustion. The fly ash, of course, is entrained in the gaseous effluent stream. Prior to emission of the effluent stream with the atmosphere, the particulate content must be reduced to meet quantitative requirements regarding the amount of particulates (pounds particulates/million Btu of coal) and the appearance of the stack plume (opacity). Fly ash typically is removed from coal combustion effluent streams by means of electrostatic precipitators. These are large and expensive pieces of equipment, the performance of which is affected by a number of factors including the electrical resistivity of the ash, the particle size, gas velocity through the precipitators, the size of the precipitator, physical condition and the like.
Typically, in coal fired utility plants the electrostatic precipitator is located either between the furnace and the air preheater, or between the air preheater and the inducted draft fan. In the former case, gas temperatures are generally between about 600.degree. F. to about 800.degree. F. and in the latter case, gas temperatures are between about 300.degree. F. to about 400.degree. F. Hence, the former precipitators are referred to as hot side precipitators, and the latter, cold side precipitators.
The low sulfur-containing coals, e.g. coals having less than about 2 weight percent sulfur, typically burned today produce a fly ash that has a higher resistivity at gas temperatures in the 300.degree. F. to 400.degree. F. range than the resistivity of fly ash from high sulfur coals. To satisfactorily remove that fly ash from utility plant effluent gas streams with a cold side precipitator requires a substantially larger and more expensive electrostatic precipitator than that required for high sulfur coals. At temperatures in the 600.degree. F. to 800.degree. F. range, however, the low sulfur coal produces a fly ash with a significantly lower resistivity, thereby decreasing the effective size and cost of the precipitator required.
The performance experience of hot side electrostatic precipitators with low rank coals often has not been as good as expected. At times the quantitative requirements on pounds particulate/million Btu have been difficult to maintain. Other times, the requirements on the quantity of particulates were easily satisfied, but opacity of the effluent was difficult to maintain at a low level. The latter performance experience has often been attributed to the low sodium content, i.e., less than about 2 weight percent Na.sub.2 O on ash, of some low rank coals. It is commonly believed, for example, that alkalis, such as sodium or potassium, strongly influence the resistivity of the fly ash. The higher the sodium content of the coal, the greater the ease of removal of the ash by electrostatic precipitators. Therefore, coals with a relatively low sodium content, for example, in the range of 0.5 to 2 weight percent Na.sub.2 O on ash, are commonly treated with dry sodium carbonate or other solid sodium-containing compounds prior to combustion to improve the operation of the hot side electrostatic precipitator. This treatment, unfortunately, is only partially effective in most cases. Additionally, excess sodium can result in severe slagging problems in the boiler. Also, most of the sodium carbonate added evaporates in the furnace and condenses on heat transfer surfaces. Thus, there remains a need for enhancing the ability of electrostatic precipitators to successfully remove particulates from gas streams generated during combustion of coal.